It kind of flows backwards - the base body flows from the things he wants it to be good at (there will be personality implications, if nothing else) which in turn flows from the types of missions he expects to be put on and the sorts of things that he expects to want/need help on, and that flows from what he's good at already. he knows a lot more of those answers than I do.
Chameleon might be good at stealth magics, but it's a little too focused - if he wanted to focus on stealth to that degree, it would be because he expected to be in a lot of situations where stealth was critically important, and that, in turn, would only make sense if he himself was good at stealth, thus eliminating the need for his familiar to do it for him. Also, while they're good at not being seen, chameleons are odd - once they are seen, they'll stick in someone's memory. You'd rather have a body (like mouse/rat/spider or maybe snake - or local equivalents thereof, depending on how that works) that will be reasonably good at the stealth, common enough to raise attention, and expected to act stealthy (say, because it's a terrified prey animal) so that the fact that it was sneaking around won't raise attention either.
Chameleon might be good at stealth magics, but it's a little too focused - if he wanted to focus on stealth to that degree, it would be because he expected to be in a lot of situations where stealth was critically important, and that, in turn, would only make sense if he himself was good at stealth, thus eliminating the need for his familiar to do it for him. Also, while they're good at not being seen, chameleons are odd - once they are seen, they'll stick in someone's memory. You'd rather have a body (like mouse/rat/spider or maybe snake - or local equivalents thereof, depending on how that works) that will be reasonably good at the stealth, common enough to raise attention, and expected to act stealthy (say, because it's a terrified prey animal) so that the fact that it was sneaking around won't raise attention either.